Bradford-based Morrisons has rejected claims by a consumer watchdog that it is among supermarkets duping customers with so-called special offers.

Which? says stores are misleading customers into paying more for groceries which are supposed to be on special offer. It said research shows that supermarkets have been selling products in offers that do not give shoppers a real deal.

Which? analysed 70,000 grocery prices and found examples of misleading multibuys which end up costing consumers more and dodgy discounts where the offer ran for much longer than the item was at the higher price.

It criticised Morrisons for promoting pizza as a special offer after selling it at a higher price for only a short time. But a Morrisons spokesman said: “The product [pizza] was sold at the higher price for 18 out of the previous 22 weeks leading up to the promotion.

“This shows that the higher price was the ‘normal’ price, which is why we disagree with Which’s conclusions and believe that this promotion complied with the OFT principles.’ Which? also found a Sainsbury’s special offer for Carex Aloe Vera & Eucalyptus Moisturising Antibacterial Handwash, where the item was priced at £1.80 for seven days, then was on offer at “was £1.80, now 90p” for 84 days.

Ocado – which next year will deliver Morrisons new online grocery service – sold a 12-pack of Beck’s beer as “was £12.19, now £9” for almost a month but had only sold the item at the higher price for three days.

Asda increased the regular price of Muller Light Greek Style Yoghurt from £1.50 to £2.18 before it went on a “two for £4” offer, costing shoppers £1 more. It also increased the price of Uncle Ben's Express Basmati Rice from £1 to £1.58 before offering for “two for £3” and then returning the rice to £1 when the offer ended. Which?, through its Make Special Offers Special campaign, wants supermarkets to put an end to misleading offers. It has also called on the Government to make rules for special offers simpler.

Richard Lloyd, Which? executive director, said “We’ve found dodgy discounts across the aisles, and with rising food prices hitting shoppers' budgets hard we think supermarkets are not playing fair.

“The stores have had long enough to sort their act out, so we're saying enough is enough, it’s time to Make Special Offers Special.”

A Department for Business spokesman said it was a criminal offence to mislead consumers on prices, including special offers.